When it was active in the 1880's, Excelsior was the largest geyser in the world. Some of its fountain- type eruptions reached 300 feet tall and 300 feet wide. It poured out so much scalding water that it was said to greatly increase the flow of the Firehole river for a half hour after the erption. The river became so hat that horses refused to cross within a quarter mile down stream. Excelsior was active as a major geyser from 1878 to 1890 (there are hints of possible eruptions in 1891 and 1901.)
After 95 years of dormancy, Excelsior awoke in 1985, erupting during a 46 hour period from September 14 to 16. These eruptions did not reach the magnitude of the 1880's eruptions. Most were relatively small at 30 feet but a few were as much as 75 feet tall and 75 feet wide. All eruptions lasted about 2 minutes with intervals of 5 to 66 minutes. During this activity, the outflow of the geyser greatly increased. Producing extra runoff channels to the river. (extra runoff channels are probably a good indicator to look at.)
Even when inactive, Excelsior still produces a huge runoff, approximately 4500 gallons per minute or 6 million gallons per day.